In recent years, theoretically tantalizing enhancements in performance of Group III-V compound, e.g., GaAs, semiconductor devices over silicon semiconductor devices spurred the development of this area of technology. However, the two main forms of synthesis processes utilized in this field present potential impediments to the rapid commercialization of this technology. Molecular beam epitaxial systems, (MBE), are not easily scalable to meet the needs of the growing Group III-V compound semiconductor device market. The MBE process presents special limitations for the expansion of the concentrator solar cell market because of the large scale quantities needed to effectively compete with conventional solar cells.
Metal organic chemical vapor deposition systems, (MOCVD), have much higher production capacities. However, the MOCVD process suffers from a poor utilization efficiency of the expensive and toxic reactant gases. In addition, large scale utilization of MOCVD systems would present difficult disposal problems for the highly toxic reaction gases now used in the fabrication process. Furthermore, the amount of reaction gases required in the fabrication process renders the technique very expensive except for specialized applications where cost competitiveness with silicon technology is not a major consideration.
We have previously described a technique named vacuum chemical epitaxy (VCE) as vacuum metal-organic-chemical-vapor-deposition at the "Space Photovoltaic Research and Technology Conference" Apr. 30-May 2, 1985. The proceedings were later published in greater detail in a paper titled "Vacuum MOCVD Fabrication of High Efficiency Cells for Multijunction Applications". The VCE technique is scalable and provides for a much higher utilization rate while minimizing the disposal problems of the toxic gases not utilized in the fabrication process. The technique was amplified upon in the Mar. 1, 1984 to Mar. 31, 1985 annual report, forwarded to SERI on July 15, 1985, titled "Research on Multibandgap Solar Cells" for SERI contract ZL-4-03123-1. FIGS. 29 and 31 illustrated a two-nozzle injection system and a multichamber reactor, respectively.
The Apr. 1-30, 1985 informal monthly communication to SERI under contract ZL-4-03123-1 addressed a seven-injection nozzle reactor chamber. More recently, this earlier work was summarized in a paper titled "Epitaxial Growth From Organometallic Sources in High Vacuum", J. Vac. Sci. Technol. B4(l), Jan/Feb. 1986, pp. 22-29. The figures alone in the J. Vac. Sci. Technology paper were presented earlier at the Electronic Materials Conference on June 19, 1985.
Additionally, the VCE process utilizing reactive gases to enable the fabrication of devices using carbon as a dopant, is described in U.S. Application Ser. No. 885,898, filed July 14, 1986. The above-mentioned application and articles are completely incorporated herein by reference for all purposes.
In spite of these publications, an improved scale-up reactor and fabrication process have not heretofore been disclosed. Thus, it would also be desirable to have an apparatus and a process which can be extended to the general use of any reactive metal organic gas and dopants in a vacuum environment to optimize the growth of uniform semiconductor layers on large area substrates while minimizing the disposal problems of the toxic reactant gases and the reaction by-products.